A generalized or localized tissue reaction occurring within minutes of an antigen-antibody reaction. Similar reactions elicited by nonimmunologic mechanisms are termed anaphylactoid reactions. The term anaphylaxis was introduced by C. R. Richet to describe acute adverse reactions which followed reinjection of an eel toxin into dogs. Instead of becoming immune (nonreacting) to the toxin, the animals developed severe and often fatal congestion and hemorrhage of the gastrointestinal tract, lungs, pleura, and endocardium.